Final answer:
The directionality of an mRNA strand is from A. left to right.
Step-by-step explanation:
The directionality of an mRNA strand refers to the direction in which it is synthesized and read. In the case of mRNA, it is synthesized from DNA during the process of transcription and read by the ribosome during translation. mRNA is synthesized in the 5' to 3' direction, which means that it is synthesized starting from the 5' end and extending towards the 3' end. Therefore, the correct answer is (a) Left to right.
During transcription, the DNA strand is used as a template to synthesize mRNA. The enzyme RNA polymerase binds to the promoter region of the DNA and starts synthesizing mRNA in the 5' to 3' direction. This means that the first nucleotide added to the mRNA strand is at the 5' end, and subsequent nucleotides are added in the same direction.
Once mRNA is synthesized, it undergoes several modifications before it can be used for translation. In the process of translation, the ribosome recognizes the mRNA and starts reading it in the 5' to 3' direction. Each set of three nucleotides, called a codon, codes for one amino acid. The ribosome reads the codons and assembles the corresponding amino acids to form a polypeptide chain, which eventually folds into a functional protein.
In the given mRNA sequence 'c a g a u g u a c g g a c g a u a a', the first nucleotide, 'c', represents the 5' end of the mRNA strand. Therefore, the directionality of this mRNA strand is from left to right, the correct answer is A. left to right (5' to 3').